Measurements of fluid refractive index are highly valued as indicia of the composition of the liquid and other characteristics thereof and the index of refraction is known to have temperature dependency so that, for making a comparison of refractive index measurements, it is necessary to modify the measured value by reducing it or referring it back to a predetermined reference temperature, e.g. 20.degree. C. ("room temperature").
Thus, it is known to provide a refractometer for determining the index of refraction of a liquid which utilizes the reflection and refraction phenomena occurring at the interface between a light-conductive path andd the liquid close to the critical angle which is a function of the index of refraction of the fluid.
Earlier refractometers utilizing this principle are relatively simple and precise instruments. In general, they may comprise an optical detector, a light source and a light-conductive structure connecting the detector with the source and of a configuration (curvature in alternate senses) such that internal reflection and refraction is induced along the light path between the input end and the output end thereof. Because the sensor responds only to the light reaching the sensor along the path and not the light which, because of the critical angle, passes into the liquid, the light intensity at the detector is a function of the critical angle and hence a measure of the index of refraction at the particular temperature of the liquid. This light intensity can be converted by the detector into an output signal whose value represents the measured index of refraction.
The means forming the light conductive path is immersed in the liquid whose index of refraction is to be measured.
The measured index of refraction with such a probe is the true index of refraction and is a function of two parameters. One parameter is the light bending characteristic of the liquid, a parameter which is dependent upon physical properties of the liquid, while the other parameter is the temperature.
The true index of refraction can seldom be utilized unless the temperature at the point of measurement is also known or ascertained. In most common cases, both values must be obtained to allow separation of the part of the index which is due to refraction proper and the physical properties of the liquid and which part may be due to temperature.
It should be apparent that comparison of refractive indices obtained in this manner, to permit deductions about relative compositions of liquids or the like, is seldom possible unless both indices have been measured at precisely the same temperature.
It is a common practice, therefore, utilizing empirically derived equations for the temperature coefficient of the index of refraction to reduce a measured value of an index of refraction at any temperature to the value of the index at a reference temperature, generally room temperature.
Thus the device referred to herein can be considered to be a retractometer capable of indicating the index of refraction of a fluid referred to or reduced to a predetermined reference temperature.
However, even utilizing such empirical formulas, it is not always possible to obtain accurate values and clearly, without complex electronic circuitry, reduced refractive index measurements cannot be obtained.
Electronic equipment, containing transfer functions, including data or the like for modifying the input signal from the detector in response to a measured temperature to yield a refractive index corrected to a reference temperature, is extremely complicated and costly, being useful only for industrial applications in which expensive equipment may be utilized readily.
There are, however, many cases in which it may be desirable to utilize refractive index measurements and in which such expensive equipment may be impractical or prohibitively costly. For example, if one wishes to determine the state of charge of a storage battery utilizing a liquid electrolyser, one may use a refractometer since the refractive index is a measure of sulfuric acid concentration in a lead acid battery and hence reflects the state of charge.
In practice, a low cost apparatus capable of conveniently and efficiently measuring the refractive index of an acid electrolyte for lead-acid storage batteries has been unknown in the art.